This invention relates to a vibratory transducer whose dimensions are selected for ease of fabrication and to avoid buckling and spurious modes of operation.
Double-bar transducer elements formed in the shape of double-ended tuning forks have been proposed for a variety of uses including use of an ultrasonic transducer with feedback control (U.S. Pat. No. 3,148,289), use as an accelerometer element (U.S. Pat. No. 3,238,789), use as a force transducer (U.S. Pat. No. 4,215,570), and use in a beat frequency oscillator (U.S. Pat. No. 2,854,581). In the latter three mentioned references, the double-bar transducer element is utilized to measure forces applied to the bars along the longitudinal axes thereof.
Among the advantages of the double-bar transducer arrangement, at least those which utilize a rigid material such as quartz, is a high mechanical Q which in turn provides high resolution. The high mechanical Q is achieved in part because the bars are caused to vibrate in 180.degree. phase opposition, with the bars being coupled together at their ends at nodes so that very little energy is lost to the mounting structure during each cycle of vibration.
One problem which has been discovered with the double-bar vibratory transducer is that a number of spurious modes of operation may develop over the operational range of the device and these spurious modes result in a lowering of the mechanical Q of the device, a shift in frequency, and a possible cessation of oscillation at the desired natural resonant frequency, i.e., transversely in a generally 180.degree. phase opposition. This desired resonant frequency changes with a change in the application of compressive or tensile forces in the longitudinal direction to the transducer, and this characteristic enables use of the device as a force transducer. However, during operation of the transducer spurious modes of operation may develop including (a) flexure or oscillation of the bars in phase in a direction normal to the plane of the transducer, and (b) flexure or oscillation of the bars 180.degree. out of phase in a direction normal to the plane of the transducer. Further, the first mentioned spurious mode has overtone frequencies in addition to the fundamental frequency. These spurious modes may be excited by the longitudinal pumping motion of the structure resulting from the bars flexing in and out, by the piezoelectric effect (assuming piezoelectric material is used) if the structure's geometry is poorly chosen, and by the nonlinear elastic behavior of the transducer material. The existence of these spurious modes of operation have not been recognized in the past. Because acoustic energy can transfer from the desired resonant mode to the spurious modes, they can result in a "glitch" or "dead" region where the transducer will not measure an applied force or, at best, it will measure it incorrectly.
In addition to the above-described spurious mode problem with double-bar vibratory transducers, another problem is that of buckling. In particular, attempted miniaturization of the transducer may result in a structure which simply cannot withstand certain compressive forces to which it may be subjected. In such cases, the transducer bends or buckles and is thereby rendered inoperative.
The above first-mentioned spurious mode (and its overtones) problem, although discussed with respect to double-bar transducers, also applies to single beam, force transducers and as described in U.S. Pat. Nos. 3,470,400 and 3,479,536, and the invention to be described hereinafter is applicable in some instances to such single beam structures.
Another factor to be considered for both double-bar transducers and single beam transducers is the need for configurations which will allow use of photolithography and chemical etching for fabrication. Such fabrication techniques provide cost advantages, miniaturization and tight dimensional control.